Making castings of aluminum-silicon alloys



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JUNIUS n. EDWARDS, or PITTSBURGH. rR'ANcIs c. ,raaiw, or oAKmoNT, AND HARRY v. CHURCHILL, F PITTSBURGH, PENNSYLVANIA. ASSIGNORS T0 ALUMm'U'm COMPANY or A'IaERIoA, roR'ATIoN OF PENNSYLVANIA.

or PITTSBURGH, PENNSYLVANIA, A coin:

MAKING CASTINGS 0F ALUMINUM-SILICON ALLOY S.

No Drawing. Application filed November T 0 all whom; it may concern.

' Be it known that we, JUNIUS D. EnwAnos, FRANCIS C. Fraser, and HARRY V. CHURCH- ILL, each a citizen of the United States of America, residing at, respectively, Pittsburgh, ()akmont, and Pittsburgh. in the county of Allegheny-and State of Pennsylvania, have invented certain new and useful Improvements in Making Castings of Aluminum-Silicon Alloys, of which the following is a. full, clear, and exact description.

This invention relates to aluminum alloys, particularly those containing silicon, and its chief object is to provide a method of making castings of such alloys which will improve the physical properties thereof, especially tensile strength and ductility. The invention is based on the discovery that the addition of sodium or potassium, or both, in metallic form, has an Important effect upon the structure of the constituents, especially in the case of alloys containing silicon in substantial amounts. p

Considerable variation is permissible in the proportions of themetals. In general, as the silicon content is increased the tensile strength of the alloy when cast in an ordinary sand mold is increased, up to a silicon content of about 10 per cent, but the elongation is decreased. For example, sand castings of an alloy containing ten parts of silicon to' ninety parts of aluminum, in the form of cylindrical bars having a test section two inches long and a half inch in diameter, show an average tensile strength of about 20,000 pounds per square inch and an elongation of about 3 per cent, whereas similar castings of ordinary commercially pure aluminum have a tensile strength of about 12,000 pounds per square inch and an elongation of about 25 per cent. By means of the present invention it is possible not only to produce a substantial increase in the tensile strength of alloys containing silicon, but also to double and even treble the elongation.

In carrying out the invention in the preferred manner an alloy of aluminum and silicon is prepared, containing from 5 to 15 per centsilicon, approximately. This can be done. for example, by stirring metallic silicon in molten aluminum at a temperature Specification of Letters Patent.

Patented Mar. 211, 1 9252.

27, 1920. Serial No. 426,796..

of about 750 to 800 C. To the alloy at this temperature the alkali metal or metals mentioned are added in metallic form and the melt stirred vigorously. Only a very small quantity of sodium or potassium is required, say about 0.1 per cent, but better results are obtained with sodium and potassium together, say about 0.05 per cent of each. For the best results the stirring .should be rapid and the cast made romptly. -The silicon content is preferably etween 6 and 15 per cent, 8 to 13 per cent being usually the best. With the first mentioned percentage, round sand-cast bars having test sections two inches long and a half-inch in diameter showed a tensile strength of about 25,000 pounds per square inch and an elon gation of from 12 to 14 per cent.

It is very desirable that the iron content be kept low, preferably less than 0.6 per cent, though in exceptional cases L5 per cent of iron may not be-too high if ductility is not of great importance.

It has been observed that if the .alloy after the addition of the alkali metal is allowed to stand in the molten state it is subject to a change which prevents the attainment of the best properties in the castings, and accordingly the cast should be made without undue delay after reaching proper pouring temperature, say 700 C. It is impossible to state definitely how long a time may elapse before casting and still realize the advantages of our invention, since that factor depends chiefly upon properties desired in the castings. With an increased amount of alkali metal a longer time may elapse without detriment to the product, but otherwise there is usually no material advantage but rather a disadvantage in em- I In some cases the addition of a meta or v of copper u metals capable of forming solid solutionsin aluminum, as for example copper and .zinc, is advantageous.. Thus the presence to about 0.5 per centhas a favorable e ect upon the tensile strength. Copper to the amount of 0.3 per cent glves the maximum improvement in tensile strength without materialdecrease of ductility. However, the copper content can be increased up to 4 or 5 Iper cent with marked improvement in the p ysical properties of the alloy if the chill method of casting or the heat treatment described and claimed in the copending applications of Junius D. Edwards and Robert S. Archer, Serial No. 426,839, and Zay Jefl'ries and Robert S. Archer, Serial No. 435,024, be followed. In the first of these applications there is described a method which consists, broadly, in filling a suitable mold with the molten alum'inum-silicon alloy and causing the latter to solidify rapidly, whereby preci itating silicon is caused to take the form 0 minute particles. The method described in the se ond of the above'applications consists generally, in the case of aluminumsilieon alloys containing copper, in heating the alloy castmg at a temperature slightly below the melting point of the eutectic until the silicon particles have been sufiicientl altered and a sufiicient amount of the inter-granular copper-rich constituent has been dissolved to materially improve the physical properties of the cast alloy. In general, when copper is present in considerable amount it is usually advisable to decrease the silicon content,--3 to 10 per cent silicon being generally suitable. The zinc may he added separately, or more or less of it (say about 1 per cent) may be introduced in the form of an alloy with the alkali metal, as hereinafter described.

Microscopic examination shows that the addition of the above-mentioned alkali metals producesa change in the structure of the alloy, this change consisting in a marked reductlon in the size of the silicon particles and a finer dispersion thereof. Thus in a sand-cast alloy containing silicon but without sodium or potassium the silicon is found in large plates or needles, whereas with the addition of either or both the metals named the silicon takes the form of very small particles, usually rounded or spherical, well distributed through the body of the eutectic. The addltlon of these alkali metals appears also to alter the composition of the eutectic alloy of the aluminum-silicon system. WlthOllt alkali metal the eutectic alloy contams about 11 to'12 per cent of silicon. Slowly solidified and untreated allovs coutamlng less than that amountbf silicon coneast of eutectic plus excess aluminum, and those containing more silicon than the amount mentioned consist of eutectic plus excess silicon. On the other hand, when the above-mentioned alkali metals are added there is a range of composition between 11 and 15 per cent silicon within which the alloy may behave as a eutectic, a hypoeutectic, or a hyper-eutectic mixture. It has also been found that the change in the eutectic composition is usually accompanied by a lowering of the freezing point, in some cases as much as 12 0.. below that of the normal aluminum-silicon eutectic.

In reduction of the article size of the minor ingredient the e ect of the. addition of these alkali metals to aluminum-nickel. alloys is similar to the effect in the case of aluminum-siliconalloys. In alloys containing cop er the addition ofsodium or potassium a ects the primary grains and the cell size, making the primary grains smaller or larger according to the amount of alkali metal added, but in general making the cell size smaller. I

In the foregoing description we have specified that the alkali metal is added 1n the metallic form, this mode of expression being employed for the purpose of exclud ing the use of the metals in the form of a salt. It is however not intended to exclude the addition of the alkali metal or metals in alloy with or in chemical combination with some other metal or metals; and as a matter of fact we have found that in some cases it is more convenient to add an alloy consisting of zincv 98 per cent and sodium 2 per cent, as explained more fully hereafter. The addition (in this manner) of alkali metal to the amount of 0.02 per cent simultaneously introduces zinc to the amount of about 1 per cent.

We have also discovered that cadmium and bismuth, to the amount of about 1 per cent or less, produce an effect similar to that obtained with sodium and potassium, but the use of cadmium and bismuth is not claimed specifically herein, as such use will form the subject of a later application.

As previously stated, there may be considerable loss attendant upon the introduc: tion of the low melting and light alkali metal in pure form into the molten aluminum-silicon alloy. This loss can be reduced and further advantages secured by the use of an alloy of sodium or potassium, or both, with some heavy metal. For example, an alloy of sodium and zinc can be prepared, which contains about 2 per cent of sodium. This alloy is quite resistant to oxidation, and

its alkali metal content does not necessitate takin any special precautions in handling it. i mce this alloy has a relatively high meltingpoint, it dissolves slowly in the melten aluminum-silicon alloy, and may be rasped in a pair of tongs andthoroughly incorporated into the alloy by stirring. Alloys of sodium or potassium, or both, with other heavy metals can be used in place of the pure alkali metals in the manner described Wherever the presence of the added amount of the alloying metal is not objec-- tionable. The choice of the heavy metal to be alloyed with the alkali metal is deter mined in part by its effect on the properties of the resulting final alloy. The amountof alkali metal alloy required depends on its alkali metal content, and the efficiency of its utilization. 'As stated above, in the case of a sodium-zinc alloy, we have found that the use of enough to introduce .02 per cent sodium was suflicient to give satisfactory results.

The term alkali metals is commonly considered to include lithium, sodium, potas sium, rubidium and caesium. Of these, only sodium and potassium are at all common or obtainable at a reasonable price and in commercial quantitiesl In many of its properties, lithium is more closely related to the metals of the alkaline earths than tosodium and potassium, and we find, as a matter of fact, that lithium does not produce the desired effect upon aluminum-silicon alloys. At the present time at least, rubidium and caesium are too rare to be commercially practicable for our purpose.

Castings made of aluminum-silicon alloys with addition of alkali metal as above described have shown themselves specially adapted for rolling and forging processes, both hot and cold. In general the fine structure produced by thealkali metal addition improves the quality of the alloys for any. purpose where increased tensile strength and ductility are desired.

What we claim is- 1. In the art of making articles of aluminum alloy containing silicon in substantial amount, the improvement comprising adding to the molten alloy an alkali metal, and

causing the alloy to solidify while suflicient alkali metal remains to favorably affect the structure of the alloy.

2. In the art of making articles of aluminum alloy containing silicon in substantial amount, the improvement comprising adding to'the molten alloy metallic sodium, and causing the alloy to solidify while suflicient of the sodium remains to favorably affect the strength and ductility of the alloy.

3. In the art of making/articles of aluminum alloy containing silicon in substantial amount, the improvement comprising adding to the molten alloy, alkali metal to the amount of 0.1 per cent approximately, and causing the alloy to solidify while sufiicient alkali metal remains to favorably affect the structure of the alloy. p

4. The method of making aluminum alloy castings of improved physical properties, comprising preparing an alloy of suitable composition, adding to the-molten alloy a relatively small amount of alkali metal, and casting the alloy while suflicient alkali metal remains to favorably affect the structure of the alloy.

5. The method ofv making aluminum alloy castings, comprising preparing an alloy of:

suitable composition contaimng a substantial amount ofsilicon, adding metallic sodium and potassium to the molten alloy, and cast-- ing the alloy while it retains enough of one or both metals to improve the physical propcastings, comprising preparing an alloy containing not less than about 3 nor more than about 15 per centsilicon, adding alkali metal to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a substantial improvement in the physical properties of the alloy.

8. The method of making aluminum alloy castings, comprising preparing an alloy containing not less than about 3 per. cent 'nor more than about 15 per cent silicon, adding metallic sodium to the molten alloy, and casting the alloy while it retains sufficient sodium to produce a substantial improvement in the physical properties of the alloy.

9. The method of making aluminum alloy castings, comprising preparing an alloy containing not less than about 3 per cent nor more than 1-5 per cent silicon, adding metallic sodium and. potassium to the molten alloy, and casting the alloy while it retains suflicient alkali metal to produce a sub- 7. The method of making aluminum alloy stantial improvement in the physical properties of the alloy.

10. The method of making aluminum I alloy castings, comprising preparing an alloy containing 8 to 13 per cent silicon, approximately, adding alkali metal to the molten altoy, and casting the alloy while it retains sufiicient alkali metal to produce a substantial improvement in the physical properties of the alloy.' I

11. The methodof making aluminum alloy castings, comprising preparing an alloy containing about 8 to 13 per cent silicon, adding alkali metal to the amount of not more than about 0.1 per cent, and casting the alloyiwhile it retains suflicient alkali metal to produce a substantial improvement in the physical properties of the alloy;

12. The method of making aluminum alloy I castings, comprising preparing an alloy con- 13. The method of making aluminum allov castings, comprising preparing an alloy containing about 8 to 13 per cent silicon, adding to the'molten alloy metallic sodium and po-. tassium in the amount of not more than about 0.1 per cent, and casting the alloy while it retains sufficient alkali metal to produce a substantial improvement in the. physical properties of the alloy.

14. The method of making aluminum alloy vastings, comprising preparlng an alloyconraining copper and about 3 to 10 per cent silicon, adding. to the molten alloy metallic sodium and potassium in the amount of not more than about 0.1 per cent, and casting the alloy while it retains sufficient alkali metal to produce a substantial improvement in the physical properties of the alloy.

15. The method of making aluminum alloy castings, comprising preparing an alloy containing copper and about 3 to 10 per cent silicon, adding metallic sodium and potassium to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a substantial improvement in the physical properties of the alloy.

16. The method of making aluminum alloy castings, comprising preparing an alloy'containing copper and about 3 to 10 per cent silicon, adding alkali metal to the amount of not more than about 0.1 per cent, and casting the alloy while it retains suflicient alkali metal to produce a substantial implrlovement in the physical properties of the a o I 1%: The method of making aluminum alloy castings, comprising preparing an alloy containing copper and not less than about 3 per cent nor more than about 10 per cent silicon, adding metallic sodium to the molten alloy, and-casting the alloy while it retains suflicient sodium to produce a substantial improvement in the physical properties of the alloy.

18. The method of making aluminum alloy castings comprising. preparing an alloy containing copper and not less than about 3 nor .more than about 10 per cent silicon, adding alkali metal to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a'substan- .tial improvement in the physical properties of the alloy.

19. The method of making aluminum alloy castings, comprising preparing an alloy of suitable composition containing copper and a substantial amount of silicon, adding to the molten alloy metallic sodium and potassium in the amount of not more than 0.05 per cent of each, approximately, and casting the alloy while it retains sufficient alkali metal to produce a substantial improvement in thevphysical properties of the alloy.

20. The method of making aluminunr alloy castings, comprising preparing an alloy of suitable composition containing copper and a substantial amount of silicon, adding metallic sodium and potassium to the molten alloy, and casting the alloy while I alloy containing not less than about 3 nor more than about 15., per cent silicon and copper in amount not exceeding about 0.5 per cent, adding metallic sodium and potassium to the molten alloy, and casting the alloy while it retains suflicient alkali metal to produce a substantial improvement in the physi cal properties of the alloy.

2.3. The method of making aluminum alloy castings, comprising preparing an alloy containing 8 to 13 per cent silicon, approximity, and copper in amount not exceeding about 0.5 per cent, adding alkali metal to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a substantial improvement in the physical properties of the alloy.

24. The method of making aluminum alloy castings, comprising preparing an amount of not more than about 0.1 per cent,

and casting the alloy whileit retains suf-, ficient alkali metal to produce a substan-' tial improvement in the physical properties of the alloy.

26. The method of making aluminum alloy castings, comprising preparing an alloy containing copper and zinc and not lessthan about 3 nor more than about 15 percent silicon, adding metallic sodium and potassium to the molten alloy, and casting the alloy While it retains sufficient alk l1 metalfto produce'a substantial improveme t in the physical properties of theialloy.

27. The method of making aluminum alloy castings, comprising preparing an alloy containing copper and zinc and. not

-,.less than about 3 nor more than about 15 per centsilicon, adding-alkali metal to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a substantial improvementjn the physical properties of the. alloy.

, 28. The method of making aluminum alloy castings, comprising preparing an" alloy of suitable compos tion containing copper and 'zinc, anda'siibstantial amount of silicon, addingmetallic sodium and po tassiumto' the molten alloy, and casting. the alloy while it retains suflicient of one or.

both metals to improve. .the physical properties of the alloy. v I 29. The method of smaki ng aluminum alloy castin comprising preparing an alloy containing zinc and about 8 to 13 pel cent silicon, adding tothe molten alloyv m'etallicsodium and potassium in the amount-of not more than about 0.1 per cent,

"improvement in t the alloy.

and casting the alloy while it retains sufproduce a substantial the physical properties of ficient alkali metal to 30. The method oi making aluminum alloy castingacomprising preparing an alloy containing zinc and 8 to 13 per cent silicon,

approximately, adding alkali metal to the molten alloy, and casting the alloy while it retains suificient alkali metal to produce asubstantial improvement inxthef physical contain'ing zinc and notproperties of the alloy.

' 31. The method of inaking aluminum ,al-

loy castings, comprising pfrepari'ng an alloy ess than about, 3

nor more than about '15 per cent silicon, adding metallic sodium and potassium to the molten alloy, and casting the alloy while it retains sufiicient alkali metal to produce a s'ubstantialimprovement in the physical propertiesof the alloy.

.32. The method ofi making aluminum al- .loyca'stings, comprising preparing an alloy containing. zinc and not less than about 3 nor more than'about 15 per cent silicon,

adding alkali metal to the molten alloy,

and casting the alloy while it retains sufli-.

ci ent alkali metal to p'roduce a substantial retains enough of one or both metals to immore than about 15 per cent silicon, and

'amount of not amount-of not more than about 0.1 per cent,

improvement in the physicalproperties of the alloy.

33. Themethod of making aluminum a1- loy castings, comprising preparing, an alloy of suitable composition containing a substantial amount of silicon and containing a substantial amount of a metal capable of forming a solid solution in the aluminum, adding metallic sodium and postassium to the molten alloy, and casting the alloy while it containing about 8 'to'13 percent silicon,

and containing a substantial amount of metal capable of forming a solid solution in the aluminum, adding alkali metal to. the more than about 0.1 per cent, and casting the alloy while it retains sufiicient alkali metalutoproduce a substantial. improvement in the physical properties of the alloy. w

- 36. The method of" making aluminum all'oy castings, comprising preparing an alloy containing about 8 to 13 per cent silicon and containing a substantial amount of metal capable otforriiing a solid solution in the aluminum, adding to the molten alloy metallic sodium and potassium in the and casting the alloy ,while it retainssuflicient' alkali metal topl'oduce a substantial improvement in the physical properties of the alloy.

37 The method oif. making aluminum .al-

loy castings, compr sing preparing an alloy of suitable composition containing a substantial amount of silicon and of a metal capable of forming-a solid solution in the. aluminum, adding alkali metal to the molten alloy, and casting the alloy while it retains enough alkali metal to improve the physical properties of the alloy.

In testimony whereof we hereunto .aflix our signatures.

'JUNIUS D. EDWARDS. ,FRANCIS C. FRARY.

HARRY V. CHURCHILL. 

